1. Field of the Invention
This invention relates to a method of producing a graphite-based conductive coating composition to be applied to the interior wall of a color picture tube.
2. Description of the Prior Art
Generally, the interior wall of the funnel portion of a color picture (cathode-ray) tube is coated with an electroconductive film for the purpose of accelerating the electron beams by applying a high voltage or for the purpose of preventing the decline of color contrast by capturing the secondary electrons emitted from the electrodes such as shadow mask fluorescent screen.
Such an electroconductive coating film needs to be firmly attached to the wall surface so that it won't peel off in use. It is also essential that this coating film be provided with a prescribed resistivity to minimize arcing.
Heretofore, these electroconductive coating films have been formed by applying a coating composition containing powdered electroconductive materials on the interior wall of the funnel portion and then subjecting the coated wall to a heat treatment. The coating composition used was prepared by dispersing a metal oxide such as titanium oxide and a small quantity of graphite in a silicate solution, the so-called water glass, in the presence of a dispersant.
However, this conventional coating composition has a serious defect in that it is very poor in dispersion stability. This is due to the fact that, although titanium oxide is dispersed uniformly in the silicate solution immediately after mixing, said compound tends to give rise to a phenomenon of aggregation several minutes to several hours after mixing and is liable to separate from the silicate solution. Use of a coating composition with nonuniformly dispersed titanium oxide does not allow formation of a fast and secure electroconductive coating film and also results in wide variations in resistivity, making it impossible to provide a stable high resistivity.
In order to prevent separation of the components of the coating material, a continuous and strong mixing and stirring operation was required in the coating work, so that the coating workability was poor. Hence it was impossible to perform the coating operation quickly and with high efficiency.